Long-term objective is knowledge of the factors that determine and regulate the metabolic exchange between mother, placenta, and fetus. The solution of important health related problems, such as detection and prevention of fetal hypoxia, malnutrition, and growth disorders, depends on the acquisition of this basic knowledge. The specific aims are: (1) to study placental oxygen transport in twin-pregnant ewes that carry hemoglobins with different oxygen affinities. (2) to study developmental changes in placental glucose transport, (3) to test the hypothesis that the serine metabolized by the placenta is primarily of fetal origin. (4) to study the effect of undernutrition on uteroplacental metabolism and placental oxygen transport, and (5) to study mechanisms of fetal lactate and pH regulation in a controlled sae of fetal hypoxia associated with a significant decrease in fetal oxygen uptake. The studies will be performed in chronic sheep preparations. The maternal-placental-fetal exchange will be quantitated by using techniques of blood flow measurement, chemical analysis of maternal arterial, uterine venous, umbilical arterial and venous blood, and tracer methodology.